1. Technical Field
The present invention relates to a recording medium which records information by varying optical characteristics of a recording film by applying an external energy, such as light and heat.
2. Background Art
Conventionally, the following have been known as recording media which have oxidation-reduction type recording films, for example.
An optical information recording medium set forth in Japanese Unexamined Patent Publication (KOKAI) No. 2-152,029 employs a laminated film, which includes a metal layer (a first layer), such as Al, Cu and Ag, and a layer (a second layer) including S or Se or mixtures thereof, as a recording film. Further, Japanese Unexamined Patent Publication (KOKAI) No. 60-99,699 discloses a recording medium which includes a carrier and a mixture layer formed on the carrier. The mixture layer includes a low melting point metal, such as one or more of In and Sn, and MoO3 and/or Ga2S3. Moreover, in the specification of U.S. Pat. No. 5,459,018, a recording medium is disclosed in which a metal, such as Al and Fe, is employed as a first substance and an oxide, such as TeO2 or In2O3, is employed as a second substance and in which these substances are mixed to make a single layer or are laminated alternately to make a laminated film.
In the optical information recording media having these recording films, the recording films are modified physically and/or chemically by applying a recording laser beam (external energy), thereby varying the optical characteristics, such as reflectance, etc. The recording media can record information by using the variations of the optical characteristics.
However, among these recording media, in those having a recording film formed by the mixture layer of the first substance and the second substance, since the first substance and the second substance are mixed, the first substance and the second substance react with each other slightly even when information is not recorded. Thus, it is easy to think of harsh chronological degradation. In the recording film in which the first substance layer and the second substance layer are laminated, since the contact area between the first substance and the second substance is small, a large energy is required when information is recorded.
It is a first object of the present invention to provide a recording medium which exhibits a high stability, and which hardly requires a large energy when in formation is recorded.
Further preferably, it is an object to provide a recording medium in which the difference between the recorded potions and the unrecorded portions is so clear that the recording medium is good in terms of recording characteristics.
Furthermore preferably, it is an object to provide a recording medium which is produced with ease.
The inventors of the present invention thought of making the recording film a laminated construction of 2 layers or more and reducing the contact area between the first substance and the second substance so that a stable recording medium is less varied chronologically. They also thought that the reaction can be carried out with less energy by melting a first reactant substance layer constituting the recording film in the recording period, and carrying out the reaction in a liquefied state.
And, the inventors thought of functioning the surface of the metal layer as a reflector surface by making the first substance from a metal of high reflectance, to obtain a recording medium of good recording characteristic in which the difference between the recorded portion and the unrecorded portion is more distinct.
Further, the inventors thought of producing a recording medium with ease by making the metal layer two or more compositional portions, or two or more phases of different crystalline states. Thus, the reaction with the second substance occurs only when a predetermined energy is applied, and the reaction does not proceed when producing.
And, regarding the constituent substances of the first layer and the second layer, constituting the recording film, the inventors made various experiments and researches. Thus, the inventor completed the present invention.
Namely, the present invention is characterized in that the first layer includes a metal and/or a substance including the metal, the metal and/or the substance generating energy by 500 kJ or more when it reacts with 1 mol of oxygen molecules (hereinafter referred to as an xe2x80x9coxygen bonding energyxe2x80x9d), and exhibiting a melting point or a decomposition temperature of from 100xc2x0 C. to 300xc2x0 C.; the second layer includes a substance which includes at least one of an oxide and a substance including oxygen as a constituent element, the oxide or the substance requiring energy by 550 kJ or less when it dissociates 1 mol of oxygen molecules (hereinafter referred to as xe2x80x9coxygen dissociating energyxe2x80x9d); and, furthermore, the first layer and the second layer are a combination which causes an exothermic reaction when an external energy is applied.
In the present invention, the first layer is made to have a melting point or a decomposition temperature in the aforementioned range, and accordingly the first layer can be liquefied with a small external energy, the reactivity of the reaction of the both layers (solid phase-liquid phase reaction) is improved. When the melting point or the decomposition temperature of the metal constituting the first layer is less than 100xc2x0 C., the heat resistance of the recorded data degrades. When it exceeds 300xc2x0 C., a large external energy is required and is unpreferable.
Further, since the reaction between the both of the layers is an exothermic reaction, the reaction does not proceed in the reverse direction and the retention characteristics of the recorded data can be secured.
When the oxygen bonding energy of the metal, constituting the first layer, is less than 500 kJ, the reactivity degrades, and the contrast deteriorates. Furthermore, when the oxygen dissociating energy of the second layer exceeds 550 kJ, the oxygen is less likely to dissociate, and the reactivity degrades unpreferably.